This invention relates to disc drive components and, more particularly, to improving the torque retention characteristics of a low profile swage mount fastener used for securing a load beam of a magnetic head suspension assembly to an actuator arm.
Disc drives are used in a variety of devices such as computers, facsimile machines, copiers and any other equipment where mass data storage is desired. Two important and costly sub assemblies in disc drives are a head suspension assembly (HSA) and a controller driven actuator assembly. Included in the HSA is a read/write head that is capable of reading digital information stored on platters in the disc drive. The HSA and actuator assembly function as a unit to precisely position the read/write head vertically adjacent a precise point on the disc drive platter, with the actuator arm directing the movement of the HSA across the face of the platter through control of an actuator arm connected to the HSA. Each sub-assembly includes precision made components manufactured to exacting standards that consequently increase the cost of such parts. Because of the complexity and cost involved in each sub-assembly, manufacturers have found that having the ability to disconnect and reconnect the HSA and actuator assembly substantially reduces costs should either assembly require rework or replacement.
To facilitate the cost effective repair and/or replacement of these assemblies, the HSA and actuator assembly are typically connected at a stacked joint defined by the distal end of the pivoting actuator arm and the proximal end of a thin elongated load beam. The opposite end of the load beam supports the read/write head that sweeps across the disc platter in response to an actuator controller. Satisfactory operation of the drive requires the read/write head to be selectively positioned vertically adjacent to selected microscopically compressed annular data tracks formed on the platter within response times on the order of milliseconds. Such response involves acceleration levels on the sweeping load beam of up to 40 Gs (gravities). Therefore, the fastener connecting the actuator arm to the load beam must have the capability to withstand considerable torque on the order of 5-10 inch-ounces or more.
Disc drive manufacturers continually endeavor to reduce the size of their disc drives to meet market demands for faster drive operating speed and increased storage capacity. To increase storage capacity, additional double-sided platters are vertically stacked with each platter having its own actuator assembly and HSA. However, adding platters to a disc drive has the undesirable effect of increasing the vertical profile of the drive. As a result, manufacturers have endeavored to miniaturize existing drive components.
One such method for reducing the overall drive size is to reduce the size of the stacked vertical joint connecting the HSA to the actuator assembly. For example, in U.S. Pat. No. 5,689,389 (Braunheim '389), the contents of which are incorporated herein by reference, a low profile swage mount fastener is used to connect the load beam of the HSA to the actuator arm of the actuator assembly. Because the swage mount fastener has a low profile, the overall height of the disc drive may be reduced, especially in drives using multiple platters and HSAs. However, a disadvantage of using a low profile swage mount fastener is that as performance demands increase, it provides less torque retention than is required to withstand the force levels imposed on the sweeping load beam.
In Braunheim '389, the torque retention characteristics of a low profile swage mount fastener were increased by modifying the internal geometry of the swage mount. However, the level of torque retention that can be achieved solely by modifying the swage mount design is limited. Without increased torque retention values, the acceleration rate a load beam can withstand is limited, which imposes an upper limit on the speed at which the read/write head can be positioned over a selected portion of the platter. This in turn will limit the overall access time a disc drive can achieve, a key parameter of disc drive performance. Accordingly, there is a shortcoming in the art in the unavailability of a low profile swage mount fastener capable of torque retention values that are significantly higher than have been previously achieved.
Moreover, when the load beam is accelerated to high speeds for positioning the read/write head to a particular location on the platter, the read/write head has a tendency to lift vertically away from the platter. If the head moves too far away from the platter, drive read/write operations are adversely effected. To minimize this affect, the HSA is "preloaded," that is, it is biased toward the platter. In preloading, the HSA is curved downwardly positioning the read/write head closer to the platter. Preloading thus creates a downward force on the head so that when the load beam is accelerated, this downward force restrains the head from lifting above a horizontal plane parallel to the platter. Generally, preloading of 3 to 4 grams of force is sufficient to achieve this result.
A present drawback in using a swage mount fastener to join the HSA to the actuator assembly is that the process of swaging causes "gram loss," i.e. it negates the beneficial effect of preloading. The effect swaging has on gram loss is measured by comparing the pre-swage gram load on the HSA versus the post-swage gram load. It has been found that an average gram loss distribution of about 0.1 to 0.3 grams occurs as a result of the swaging process. Accordingly, it would be desirable to provide a low profile swage mount fastener in which gram loss could be reduced while increasing torque retention values.